Describe the principal structural features of turtles that would distinguish them from any other reptilian order.

The principal structureal features of turtles that distinguish them from any other reptilian order are that they are enclosed in shells consisting of a dorsal carapace and a ventral plastron. The shell is also composed of two layers: an outer horny layer of keratin and an inner layer of bone. The bony layer is a fusion of ribs, vertebrae, and many dermally ossifying elements. What is unique about turtles among other vertebrae is turtle limbs and limb girdles are located inside the ribs. Lacking teeth, a turtles jaw is provided with tough, horny plates for gripping food.